1. Field of the Invention
The present invention relates to disturbance compensation in an actuator, and more particularly to a method and device for compensating for a disturbance caused in an actuator of a magnetic disk drive.
2. Description of the Related Art
In general, a magnetic disk drive includes a tracking control function for a magnetic head, in order to accurately record and reproduce data. However, disturbances arise from the external environment when controlling a servo, and these disturbances become a main cause of reduced accuracy of the tracking control. These disturbances may be broadly classified into bias force disturbances, caused by the elastic character of the FPC (Flexible Printed Circuit) cable connected to the actuator, and runout disturbances, caused by repetitive or non-repetitive runout of the magnetic disk. Runout disturbance is generally caused by transversal irregularities (i.e., poor flatness) of the disk, which may arise when the center of the disk is not precisely aligned with the spindle motor axle, or when the disk is defectively mounted.
A known control algorithm has been proposed to effectively compensate for bias disturbances and runout disturbances, thereby providing precise control of the magnetic head and accurate recording and reproduction of data. According to this approach, the actuator generates an actual position value x of the magnetic head on the track. An estimating device receives the actual position value x and the control signal input u, to generate the estimated values x and f.sub.B in accordance with the following equation: ##EQU1## where x represents an estimated position value, v represents an estimated velocity value, f.sub.B represents an estimated f.sub.B value, 1.sub.1, 1.sub.2, 1.sub.3 represent gains of the estimating device 14, and ".cndot." represents differentiation.
It is very important to compensate for these disturbances effectively in order to achieve accurate control of the magnetic head and therefore accurate data recording and reproduction. Therefore, it is necessary to identify the effects the disturbances cause when the magnetic head follows a track for reading or writing data.
The bias force disturbance f.sub.B may be modeled by a DC signal, in view of its frequency. It can therefore be appreciated that effective compensation can be provided for the bias force disturbance f.sub.B if the controller further includes an integral controller or if the estimating device calculates the estimated values based on Equation (1). On the other hand, the runout disturbance f.sub.R, must be modeled by a sinusoidal wave signal having a frequency corresponding to the angular frequency of the rotating disk. Therefore, it may not be possible to effectively compensate for the runout disturbance f.sub.R, even if the controller further includes an integral controller or if the estimating device calculates the estimated values based on Equation (1). This is because the runout disturbance f.sub.R does not enter into Equation (1).
A countermeasure for this problem is to expand the bandwidth of the entire control system by increasing the gain of the controller. This approach does not offer a real solution, though, because track pitch for state-of-the-art designs is on a decreasing trend for high data density devices. It is expected that in the future track density will increase to about six times the current track density. This implies that the track pitch will decrease to 1/6 of the current track pitch. Consequently, a control algorithm is required that will effectively decrease the influence of disturbances by six-fold.
For such a control algorithm, the bandwidth should be expanded by a factor of .sqroot.6 compared with existing bandwidths. However, the bandwidths of actuators currently in use are limited by input terminal saturation, so that there is a limitation on expanding the bandwidth of the system by increasing the gain of the controller. More significantly, if the bandwidth is expanded, the system may become unstable.
A demand therefore exists for a control algorithm which will compensate for the runout disturbance effectively, without expanding the bandwidth of the control system. Another known tracking control system does exist that will compensate both for the bias force disturbance and for the runout disturbance. This system employs a device for estimating both runout disturbance and bias force disturbance. It should be noted that the actual position value x (i.e., the relative position value of the magnetic head with respect to the center line of the track) can be directly measured by means of the tracking control of the magnetic head. Thus, the dynamic characteristics of the control system with respect to the actual position value x may be expressed as follows: EQU Jx=K.sub.t u-f.sub.R -f.sub.B (2)
Here, the bias force disturbance f.sub.B may be modeled as a DC signal, again due to its frequency, and the runout disturbance f.sub.R may be modeled as a sinusoidal wave signal having a frequency related to the angular frequency of the disk. Thus, f.sub.B =0, and f.sub.R =-.omega..sub.0.sup.2 f.sub.R, where .omega..sub.0 is the angular frequency (expressed in, for example, radians per second) of the disk.
It follows that, by considering the actual position value x of the magnetic head and the disturbances f.sub.B and f.sub.R as the state variables, the system of Equation (1) becomes ##EQU2##
It is noted that Equation (3) is a fifth-order linear system and therefore allows the estimating device used to generate bias and disturbance force values to be readily incorporated: ##EQU3## Here, ".cndot..cndot." denotes second-order differentiation.
A further method of compensating for the runout disturbance is disclosed in U.S. Pat. No. 5,404,253, issued on Apr. 4, 1995. In the system of this reference, the bias force disturbance f.sub.B and the runout disturbance f.sub.R are modeled, respectively, by a DC signal and a sinusoidal wave signal having a frequency equal to the angular frequency .omega..sub.0 of the disk. This discrete value control system is also a fifth-order system in which the estimating device estimates the head position x, the head velocity v, the bias force disturbance f.sub.B, and the runout disturbance f.sub.R. Equation (5) expresses a model of this system: ##EQU4## where a, b.sub.1, and b.sub.2 are system parameters determined when the actuator 48 is discretized, .omega..sub.0 is the angular frequency of the disk, T is the sampling time, and 1.sub.1, 1.sub.2, 1.sub.3, 1.sub.4, and 1.sub.5 are gains of the estimating device. It is noted that inclusion of the estimating device here also leads to a fifth-order linear system.
The control input signal u(n), determined according to the estimated values x(n) , v(n) ,f.sub.B (n), f.sub.R (n) generated by the estimating device of this system, has the following congruency (i.e., defining) equation: EQU u(n).congruent.K.sub.1 {x*-x(n)}-K.sub.2 v(n)-f.sub.B (n)f.sub.R (n) (6)
Here, the design parameters include the state feedback gains K.sub.1 and K.sub.2 and the gains 1.sub.1, 1.sub.2, 1.sub.3, 1.sub.4 and 1.sub.5 of the estimating device, which may be independently determined by means of a pole-placement design method.
The discrete value control system of Equation (5) has certain advantages, but its seven design parameters are a definite disadvantage: this large number makes the design process complicated. Moreover, because the overall system behaves as a seventh-order system, overshoot may increase during transient response. Computational speed may be undesirably slow, also, because the estimating device must generate values for five variables.